Pigment compositions including all-dielectric pigment flakes are, generally, substantially transparent, allowing overprinting. Such pigment compositions may be coated over an image or an underlying color, so that the image or the underlying color may be seen through the pigment compositions. In contrast, pigment compositions including conventional metal-dielectric pigment flakes, e.g., Fabry-Perot-type pigment flakes, may dull or obscure the image or underlying color.
Pigment compositions that are both chromatic and substantially transparent may be formed by using chromatic all-dielectric pigment flakes. For example, chromatic all-dielectric pigment flakes may include alternating layers of a high-index dielectric material and a low-index dielectric material, i.e., a dichroic stack. As a result of thin-film interference, a particular color component of light incident on such a pigment flake is reflected by the dielectric layers and another color component is transmitted, so that the pigment flake appears to have a color referred to as a background color. In other words, the dielectric layers provide a background color. Examples of chromatic all-dielectric pigment flakes are disclosed in U.S. Pat. No. 7,238,424 to Raksha, et al., issued on Jul. 3, 2007, in U.S. Pat. No. 6,524,381 to Phillips, et al., issued on Feb. 25, 2003, in U.S. Pat. No. 5,569,535 to Phillips, et al., issued on Oct. 29, 1996, and in U.S. Pat. No. 5,059,245 to Phillips, et al., issued on Oct. 22, 1991, which are incorporated herein by reference.
Pigment compositions that are both diffractive and substantially transparent may be formed by using all-dielectric diffractive pigment flakes. For example, all-dielectric diffractive pigment flakes may include at least one dielectric layer including a diffractive structure, e.g., a diffraction grating. Light incident on such a pigment flake is diffracted into its color components, i.e., angularly dispersed according to wavelength, by the diffractive structure, so that the pigment flake appears to have different colors at different viewing angles, which is referred to as a diffractive effect. In other words, the diffractive structure provides a diffractive effect. Examples of all-dielectric diffractive pigment flakes are disclosed in U.S. Pat. No. 6,815,065 to Argoitia, et al., issued on Nov. 9, 2004, which is incorporated herein by reference.
In some instances, an achromatic diffractive pigment composition providing a white, i.e., color-neutral, background color and a white diffractive effect is desirable. The neutral white background color substantially preserves the underlying color of an object coated with the pigment composition, and the neutral white diffractive effect exhibits essentially all colors of the rainbow at different viewing angles over the underlying color. An example of an achromatic all-dielectric pigment flake providing a white background color and a white diffractive effect is disclosed in U.S. Pat. No. 6,815,065. However, this pigment flake has a relatively complex optical design requiring several different layer thicknesses.
Achromatic diffractive pigment compositions including conventional metal-dielectric pigment flakes, generally, provide a gray or black background color and an iridescent diffractive effect. Examples of achromatic diffractive metal-dielectric pigment flakes are disclosed in U.S. Pat. No. 6,749,936 to Argoitia, et al., issued on Jun. 15, 2004, in U.S. Pat. No. 6,749,777 Argoitia, et al., issued on Jun. 15, 2004, and in U.S. Pat. No. 6,692,830 to Argoitia, et al., issued on Feb. 17, 2004, which are incorporated herein by reference. Examples of chromatic diffractive metal-dielectric pigment flakes are disclosed in U.S. Pat. No. 6,902,807 to Argoitia, et al., issued on Jun. 7, 2005, and in U.S. Pat. No. 6,841,238 to Argoitia, et al., issued on Jan. 11, 2005, which are incorporated herein by reference.